The present invention relates to an infrared gas analyser, especially for deployment as a gas detector for leak detection based on the sampling probe principle, with a vessel (test vessel), through which the test gas flows, and an infrared light source which produces an infrared light that shines through the test vessel and with a detector that facilitates the measurement of the infrared light absorption in the test gas. Moreover, the present invention relates to a method for operating a gas analyser of this kind.
The principle of leak detection employing a sampling probe is frequently employed in the case of test samples which contain refrigerants or hydrocarbons (for example, condensers in refrigeration units). In this kind of leak detection, the media present in the test sample serve as the test or measurement gases. If a leak is present, small quantities of the test gas in each case will enter via the hose of the sampling probe into a gas detector which is so designed that it is capable of detecting these gases.
In relying on the sampling probe principle to detect leaks there exists the problem that at the probe tip not only test gases escaping from a possibly existing leak are taken in, but also gases may be taken in which are present in the environment of the probe""s tip. If the latter already contain the test gas at a low concentration, for example, resulting from previously determined leaks or from the filling station of a production line, these will also be recorded by the gas detector. At high background levels for the test gas this may result in incorrect measurements, i.e. test samples which are leak tight are xe2x80x9cdetectedxe2x80x9d incorrectly as leaky.
Commonly, test gas detectors in leak detectors employ mass spectrometers. Mass spectrometers do offer a high detection sensitivity, but are costly and not particularly rugged.
The test gases mentioned above are active in the infrared range so that also an infrared gas analyser might be employed as the gas detector. The method of infrared absorption spectroscopy is basically known as a method of analysing gases (see, for example, Hansel/Neumann xe2x80x9cPhysikxe2x80x9d, Spektrum Akademie Verlag, Heidelberg). Spectrographs as detailed in this reference, however, are involved and sensitive instruments, not suited for everyday use in industry. Moreover, also known are industrial instruments (for example, MULTIWARN from the company Drxc3xa4ger) which, however, are by one or two orders of magnitude too insensitive for leak detection purposes and/or are, as to their response time, considerably too slow (30 to 60 seconds).
It is the task of the present invention to create an infrared gas analyser suitable for everyday work and which is cost-effective, which in particular may be employed as the gas detector in a leak detector. Moreover, by employing the infrared gas analyser, the aforementioned incorrect measurements to which the sampling probe principle is prone, shall be avoided.
This task is solved by the present invention for an infrared gas analyser of the aforementioned kind in that, besides the test vessel through which the test gas flows, there is present a reference vessel through which the reference gas flows which is taken in from the environment of the point at which the test gas is taken in, and where the same or a further source of light as well as the same detector is allocated to the reference vessel, that means are present effecting a modulation of the infrared light shining through the vessels, and that means for producing the measured values are present which allow the consideration of the background signals obtained by measuring the absorption of the infrared light in the gas of the reference vessel in the case of the signals which are obtained by measuring the infrared absorption in the test vessel. In that not only the test gas, but also the reference gas coming from the environment of the probe tip is analysed, all background concentrations in the test gas or of interfering other gases can be taken into account in the formation of the measured values. In the simplest case, the signals obtained by analysing the reference gas are subtracted from the signals obtained by analysing the test gas.
It is important that only a single infrared detector be employed, since the noise contributions from two different detectors are not correlated and will not vanish when subtracted. The use of a common source of infrared light or two separate light sources (one for each vessel) is possible. It is of significance that means be present which allow a modulation of the infrared light shining through the vessels, as otherwise the use of only a single detector would not be possible.